A fixed point-to-point radio link is a two-way communication system designed for communication between two fixed locations, each location comprising at least a transmitter unit and receiver unit, i.e., a transceiver. The transceiver is often equipped with, or connectable to, at least one antenna. Fixed point-to-point radio links are commonly deployed in networks for cellular backhaul, and are therefore often subject to strict requirements on performance, e.g., in terms of allowed bit error rate and link availability. This complicates the design of transceivers for fixed point-to-point radio links compared to, e.g., design of transceivers used for cellular access, which have different requirements on availability and bit error rates.
A receiver in a fixed point-to-point radio link transceiver generally comprises non-linear components such as low noise amplifiers and mixers. These components will add non-linear distortion to a received signal. The amount of non-linear distortion added depends, among other things, on the input signal level to the non-linear elements. This is especially true when the input signal level surpasses a saturation level of a non-linear component, in which case a large amount of signal distortion can be expected to occur in the received signal. It is therefore crucial to avoid saturating any of the non-linear elements in a transceiver, and to keep input signal levels below the saturation level of the receiver at all times.
Receiver non-linearity can be alleviated by designing power amplifiers with higher saturation level. This will however lead to a less power efficient system and can also affect the receiver noise figure in a negative way. Thus, analogue receivers with high linearity often suffer from high power consumption and a sub-optimal noise performance, which is a drawback in many fixed point-to-point radio links.
Receiver non-linearity can also be alleviated by using linearization techniques at the receiver. Receiver linearization can be analogue, but a more common solution is digital linearization. Most linearization techniques, however, introduce complexity into the communication system. Thus, a system using digital linearization techniques often suffers from high system complexity, which is a drawback in many fixed point-to-point radio links.
The dynamic range of a receiver is characterized by the difference between the receiver noise floor and the receiver saturation level, this dynamic range should be as large as possible in order to ensure good transceiver performance. The receiver noise figure determines the level of the receiver noise floor, the larger the noise figure, the higher the noise floor level.
The upper limit of the dynamic range, i.e. the receiver saturation level, can be improved by increasing the power consumption in the receiver. Hence, by using, e.g., power amplifiers with increased saturation level the receiver can tolerate higher input signal levels. This however leads to less power efficient solutions, which is a drawback.
The radio propagation channel over the radio link between a transmitter and a far end receiver often includes fading phenomena. This fading alters the attenuation which affects a transmitted radio signal. Fading caused by, e.g., rain and multipath propagation usually results in increased channel attenuation. This type of fading therefore decreases the input power in a receiver, and is usually referred to as down fading.
Other types of fading include ducting, where the radio propagation channel forms a wave guide. This phenomenon decreases the attenuation of the radio propagation channel, and results in increased received power at a receiver. Phenomena such as ducting are commonly referred to as up fading.
When installing a fixed point-to-point radio link all the aspects described above, i.e., receiver dynamic range and varying radio channel attenuation due to both down and up fading, will limit the radio link hop distance. This is because a certain fade margin must be applied to the radio link budget due to rain and multipath fading. The receiver dynamic range, or rather the saturated power level of the receiver and receiver noise floor, will however limit the amount of fade margin that can be used.